Allow to compare records with equal?

[jcubic]

equal? on the same records (not same instances) should return true:

(define-record-type <pare>
  (kons x y)
  pare?
  (x kar set-kar!)
  (y kdr set-kdr!))

(equal? (kons 1 2) (kons 1 2))
;; ==> #f should be #t

This should be done after #270